The effects of flow patterns on the permeate flux in a modified direct contact membrane distillation (DCMD) module, in that there were some concaves or convexes cross the flow direction, was studied in this work. The operating parameters included temperature difference, feed flow rate, module inclination angle, gas flow rate. The DCMD experiment was conducted in a flat sheet module with using 0.2 µm pore size polytetrafluoroethylene (PTFE) membrane. The flow patterns was Simulated by computational fluid dynamics(CFD) software.
The experimental results show that increasing the temperature difference will increase the permeate flux, but also heighten the polarization phenomena. Increasing the feed flow rate and the gas flow rate can reduced the polarization phenomena. As the membrane of inclination was changed from the horizontal (flow below membrane), the permeate flux increased, and reached a maximum at about 45°, and the enhancement in flux is significant in the two-phase flow system.
The results of modified DCMD show that the concaves in the feed channel can cause turbulence to feed stream and enhance flux significantly. And the convexes in the feed channel will increase the shear stress on the membrane, thus, effectively reduce the polarization phenomena and increase the permeate flux.
The distillate fluxes of DCMD were well predicted by adopting the Knudsen-molecular diffusion in series based on Dusty-Gas model for in desalination of seawater in which the seawater was assumed to be equivalent to 3.4wt% of NaCl solution. The theoretical calculations showed that the temperature polarization coefficients were in the range of 0.5 to 0.65 that was reasonable for DCMD operation; and the concentration polarization coefficients increased significantly as the temperature difference increased or the flow rate decreased. The simulated of CFD results shows that the modified modules with concaves can cause turbulence to feed stream, and the convexes in the feed channel will increase the shear stress on the membrane surface, and thus effectively reduce the polarization phenomena.